1. Field of Disclosure
This disclosure describes novel initiating systems for the free radical and controlled radical polymerization of alkenes, and especially fluorine substituted alkenes. The polymerization is initiated directly from alkyl halides at any temperature, including room temperature under light conditions such as UV, laser or visible white light (e.g., regular fluorescent bulb). The polymers also contain halide chain ends which allow the synthesis of block copolymers.
2. Discussion of the Background Art
Fluorinated (co)polymers are fundamental specialty materials endowed with wide morphological versatility, high thermal/chemical/ageing/weather resistance, low surface energy, dielectric constant, flammability, refractive index, and moisture absorption. Applications for fluorinated (co)polymers include paints, coatings, pipe liners, transmission fluids, O-rings, fuel cell membranes, antifouling layers, optical fibers and high power capacitors. The properties, and thus the applications, of fluorinated (co)polymers are related to their molecular weights, polydispersity and the like, making their precise synthesis very relevant. However, while controlled radical polymerizations (CRPs) have recently seen remarkable developments, and atom transfer radical polymerization (ATRP), nitroxide or reversible addition fragmentation (RAFT) methods have proven very successful for acrylates or styrenes, the applicability of these methods for very reactive main chain fluorinated alkenes (such as vinylidene fluoride (VDF), hexafluoropropene (HFP), tetrafluoroethylene (TFE), and the like, still awaits demonstration.
Consequently, because (co)polymers of main chain fluorinated monomers (e.g., vinylidene fluoride (VDF), hexafluoropropene (HFP), tetrafluoroethylene (TFE), trifluorochloroethylene (TFCE), etc., are industrially significant, the study of their CRP, and the synthesis of complex polymer architectures thereby derived, is an important area of inquiry. Conversely, such polymerizations are challenging on laboratory scale, as VDF boils at −83° C. Typical telomerizations/polymerizations of fluorinated monomers such as VDF are carried out at a temperature of >80-100° C. and require high-pressure metal reactors.
Accordingly, by contrast to styrene/acrylate CRPs which are conveniently sampled on a 1 g scale, kinetic studies of VDF polymerizations involve many one-data-point experiments. This is very time-consuming and expensive due to the typical unavailability (in a research lab) of a large number of costly metal reactors which, moreover, require tens of grams of monomer. Thus, the development of methods that would allow small scale (e.g., 1 g) polymerizations at room temperature (RT) in inexpensive high-pressure glass tubes would be highly desirable. Those methods could be adapted for fast screening of a wide range of catalysts and reaction conditions, and moreover, also take advantage of photochemistry.
However, while VDF telomers (degree of polymerization (DP)=1-3) may be obtained at high temperatures (>100° C.) from transition metal salts and polyhalides using redox catalysis, there are no reports on metal-mediated VDF polymerizations. VDF polymerization at room temperature was shown to proceed with alkyl boron/O2 in a poorly controlled, free radical manner where block copolymer synthesis was not possible.
The most successful approach to date to the CRP of VDF and the like is the uncatalyzed iodine degenerative transfer (IDT: Pn•+Pm−I⇄Pn−I+Pm•), one of the oldest CRP methods, and the first implemented industrially, which emerged from research on free radical VDF telomerizations with polyhalides, especially perfluorinated mono and/or diiodo chain transfer (CT) agents (e.g., CF3—I, CF3—(CF2)3—I, CF3—(CF2)5—I, (CF3)2CF—I, I—(CF2)4-6—I, HCF2CF2CH2—I, C6F13CH2CF2—I, and even RF—CH2—CH2—I), the synthesis of which, as well as derivatization of PVDF—I chain ends, is understood. Modeling and kinetic investigations also revealed the contributions of the structure of the CT agent, of side reactions and of monomer addition mode (1, 2- vs. 2, 1-), to the degree of living polymerization.
In all current IDT-VDF-CRPs, an external radical source (e.g., t-butyl peroxide) is always required, as direct initiation from perfluoro halides or alkyl halides is not available. However, direct initiation from e.g. a halide is very important in the precise synthesis of block or graft copolymers based on fluorinated monomers where such a system would otherwise inevitably form a mixture of homopolymers and copolymers. Therefore, availability of direct halide initiation would be highly valuable.
However, while 1:1 additions of perfluoroalkyl radicals to nonfluorinated alkenes with Cu, Zn, Pd, SnCl2/CH3COOAg, Cp2TiCl, PPh3, AIBN, or (NH4)2S2O8/HCOONa occurs easily, metal catalyzed addition of such electrophilic perfluororadicals to electrophilic fluorinated alkenes (FMs) at temperatures <100° C. and especially at RT is conspicuously absent from the literature. Consequently, the ability to carry out such reactions under mild conditions would be of great synthetic use, in view of the great demand for trifluoromethylations and in the initiation of the CRP of FMs.
VDF telomerizations under high power UV are available; however, there are no reports on VDF polymerizations under regular visible light. Moreover, the concept of initiating radical polymerizations using Mn2(CO)10 and regular alkyl halides does exist in the literature. However, this concept was not applied to perfluoroalkyl halides (which have a completely different reactivity) and, moreover, there was no indication in these applications that such halides would add to main chain fluorinated monomers such as VDF.
Thus, a need exists for methods which allow small-scale polymerizations of fluorinated alkenes for, e.g., catalyst screening experimentation. A need also exists for methods which allow polymerizations of fluorinated alkenes at RT and ambient or slightly elevated pressures. A need further exists for methods which allow polymerizations of fluorinated alkenes under regular visible light. Finally, a need exists for methods which allow polymerizations of fluorinated alkenes from direct halide initiation.